Disk spring contacts, brush contacts, and clip contacts have been mainly used for electric contacts, such as connectors, switches, and terminals. For parts of the contacts, use is made, in many cases, of a composite material for contacts, which is composed of a substrate, such as a copper alloy or stainless steel, which is excellent in corrosion resistance and mechanical properties, with the substrate being coated with silver, which is excellent in electrical characteristics and solderability.
Among the composite materials for contacts, those using stainless steel for the substrate are able to make contacts of small size, since they are excellent in mechanical characteristics and fatigue life, as compared with composite materials for contacts using a copper alloy for the substrate. Thus, the composite materials for contacts using stainless steel for the substrate are used for movable contacts, such as a tactile push switch and a sensing switch, that are required to have a long service life. In recent years, the composite materials are used, in many cases, for push buttons for mobile phones, in which the number of actions of such the switches is drastically increasing, due to diversification of email functions and Internet functions. Then, there is a demand for a movable contact part having a longer service life.
Since a composite material for contacts using stainless steel for the substrate allows size reduction of movable contact parts, as compared with a composite material for contacts using a copper alloy for the substrate, the size of switches can be reduced, and the number of actions thereof can be further increased. However, the contact pressure of such a switch becomes higher, resulting in a problem of a shortened contact service life, due to wear of the silver coated on the movable contact part.
For example, as a composite material for contacts obtained by coating a stainless steel strip with silver or a silver alloy, use is made, in many cases, of a composite material provided with nickel plating as an undercoat on the substrate (for example, see Patent Literature 1). However, when such a stainless steel strip is used for the switch, silver at the portion to be contacted is peeled off, due to wear as the number of actions of the switch increases. As a result, the nickel plating layer of an undercoat on the substrate is exposed to the air, which increases contact resistance, and failures ascribed to mal-continuity become evident. In particular, this phenomenon is liable to occur in dome-shaped movable contact parts having a small diameter, which has been a crucial technical problem for further reducing the size of the switch.
In order to solve this problem, there is proposed a composite material for contacts provided with nickel plating and palladium plating in this order on the substrate, and provided thereon with gold plating (see, for example, Patent Literature 2). However, since a coating of the palladium plating is hard or rigid, there is a problem that when the number of actions of the switch increases, cracks are apt to occur.
Further, there is proposed a composite material provided with nickel plating, copper plating, nickel plating, and gold plating, in this order on a stainless steel substrate, in order to improve electrical conductivity (see Patent Literature 3). However, although nickel plating itself is excellent in corrosion resistance, cracks occur in some cases at the nickel plating layer between the copper plating layer and the gold plating layer upon bending, due to the hardness of the nickel plating, to result in a problem of deterioration of corrosion resistance by making the copper plating layer expose to the air.
Further, as a technique in order to improve the contact service life, there is proposed a composite material provided with nickel plating, copper plating, and silver plating, in this order on a stainless steel substrate (see Patent Literatures 4 to 6). In those techniques, attempts have been made to improve the contact service life. As a result, when measuring the initial contact resistance value after a heat treatment (for example, for 5 minutes at a temperature of 260° C.) simulating soldering at the time of forming a contact module, and the contact resistance value after a heat treatment (for example, for one hour at a temperature of 200° C.) simulating a keystroke test, many of those were found to be at an inadequate level to be used as manufactured products, because the contact resistance values after the heat treatments were so high. This implies that when the materials are incorporated into manufactured products, the percent defective would become high. Thus, it is assumed that only by forming a nickel underlying layer, an intermediate copper layer, and a silver outermost layer, in this order at the respective predetermined thickness on a stainless steel substrate, the contact characteristics or contact service life after thermal hysteresis are unsatisfactory.
Further, as a technique in order to improve the contact service life, there is provided a material for electric contacts in which the surface of a strip material composed of copper or a copper alloy is coated with a layer composed of silver or a silver alloy, characterized in that the grain size of the silver or silver alloy is 5 μm or greater as the average value; and there is also disclosed a method of producing a material for electric contacts, characterized by including: forming a plating layer of silver or a silver alloy on the surface of a strip material composed of copper or a copper alloy, and then conducting a heat treatment at a temperature of 400° C. or higher under a non-oxidative gas atmosphere (Patent Literature 7). However, it is found that, when the composite material for contacts obtained by coating a stainless steel strip with silver or a silver alloy is subjected to the heat treatment at 400° C. or higher, in order to control the grain size of the silver or silver alloy to be 5 μm or greater, the spring characteristics of the stainless steel strip are deteriorated, and the composite material may not be applied as a material for movable contacts. Furthermore, nickel or cobalt, or a nickel alloy or a cobalt alloy is used in the intermediate layer, and a configuration in which a copper component is present in the intermediate layer as an upper layer of the underlying layer is not disclosed.